Electrolytic capacitor and electrode foil used for electrolytic capacitor

ABSTRACT

There are provided a solid electrolytic capacitor having a low ESR characteristic and an electrode foil to be used for this electrolytic capacitor. In accordance with the present invention, a capacitor element uses an anode foil with a post-etching void factor of not more than 51% or a cathode foil with another post-etching void factor of not more than 44%, wherein the capacitor element contains a solvent consisting mainly of a water and a water-soluble metal complex being bonded with phosphoric acid ions, whereby the electrode foil increases in conductivity and decreases in resistivity, while an electrolytic solution thereof has a low resistivity, to thereby obtain the solid electrolytic capacitor having the low ESR characteristic and the electrode foil to be used therefor.

TECHNICAL FIELD

[0001] The present invention relates to an electrolytic capacitor and anelectrode foil used for an electrolytic capacitor.

BACKGROUND ART

[0002] An aluminum electrolytic capacitor generally has the followingconfiguration. Namely, a stripe-shaped high-purity aluminum foil issubjected to a chemical or electrochemical etching process forincreasing a surface-area thereof. The surface-area-increased aluminumfoil is then subjected to a conversion treatment in a conversionsolution such as an ammonium borate solution, so as to form an oxidefilm on a surface of the aluminum foil, thereby forming an anode foil. Acathode foil is similarly formed of a surface-area-increased high-purityaluminum foil. The anode foil and the cathode foil are laminated througha separator. This lamination structure is further rolled to form acapacitor element. This capacitor element is further impregnated with anelectrolytic solution and then contained in a cylinder-shapedbottom-closed metal outer case. Further, a scaling member made ofelastic rubber is also contained from the opening edge of the outercase. Furthermore, the opening edge of the outer case is scaled by aclosing process to form an aluminum electrolytic capacitor.

[0003] As electrolytic solutions to be impregnated into the capacitorelement of a small and low-pressure aluminum electrolytic capacitor,there have been conventionally known the electrolytic solutioncomprising a main solvent of ethylene glycol and a solute of an ammoniumsalt such as an adipic acid and a benzoic acid, and other electrolyticsolutions comprising another main solvent of γ-butyrolactone and anothersolute of a quaternary cyclic amidinium salt such as a phthalic acid anda maleic acid.

[0004] By the way, electronic information devices have been digitalizedin recent years, and the increase in driving frequency of microprocessorunits (MPU) as a core part of these electronic information devices hasbeen in progress, resulting in an increase in electric powerconsumption, and raising a remarkable problem of reliability caused byheat generation. As a countermeasure against this, attempts to reducethe driving voltage have been made. As a circuit for supplying a highlyaccurate electric power to the microprocessor, a DC-DC converter socalled as a voltage regulator module (VRM) has widely been used. For anoutput-side capacitor, a large number of capacitors with a lowequivalent series resistance (ESR) are used for preventing any voltagedrop. As a capacitor having this low ESR characteristic, a solidelectrolytic capacitor using a solid electrolyte has been practiced inuse and widely used as a capacitor suitable for these purposes.

[0005] The increase in driving frequency of the microprocessor has beenremarkable, however, with increasing the power consumption. In order torespond to that, the increase of the power supplied from the capacitorhas been requested for preventing any voltage drop. In other words, alarge power supply must be made in a short time, for which purpose theabove-mentioned solid electrolytic capacitor is needed to not only beincreased in capacity and decreased in size and voltage but also havethe ESR characteristic lower than ever,

[0006] Attempts have been made to further reduce the resistivity of theelectrolytic solution by including a large amount of water in theelectrolytic solution. Despite the low resistivity of the electrolyte,however, such electrolytic capacitor has other problems with aninsufficient effect of reducing the ESR thereof, and a non-good agingproperty.

[0007] As described above, there is a limitation to the reduction of theESR of the capacitor by improving the electrolytic solution to be usedfor electrolytic capacitor, and a further reduction of the ESR remainsas a difficult problem.

[0008] The present invention was made to solve the above-describedproblem and provides an electrolytic capacitor with a realized low-ESRand an electrode foil to be used for electrolytic capacitor

DISCLOSURE OF THE INVENTION

[0009] In accordance with the electrolytic capacitor of the presentinvention, a capacitor element is provided with an anode foil of anetched foil of the void factor of not more than 51% contains a bondedmaterial formed by bonding a water-soluble metal complex to phosphoricacid ions and a solvent consisting mainly of water Namely, the etchedfoil of the void factor of not more than 51% is subjected to aconversion treatment or an anodic oxidation to obtain the anode foil ofthe present invention. By setting the void factor of the etched foil at51% or less, the anode foil increases in conductivity and decreases inresistivity, to thereby achieve, in combination with the electrolyticsolution using the solvent consisting mainly of water, an electrolyticcapacitor having an unprecedentedly low ESR characteristic. If the voidfactor exceeds 51%, then the anode foil increases in resistivity, and adesirable ESR characteristic is no longer obtainable. Further, if thevoid factor is 20% or more, a high capacity is obtained and thus this ispreferable. The void factor of the etched foil in this case is definedto be a value calculated by dividing the volume of voids the etched foilby the apparent volume of the etched foil.

[0010] Usually, if a solvent consisting mainly of water is used for anelectrolytic solution, then a problem is raised with a remarkabledeterioration in hydration of the electrode foil, which causes furtherproblems with a deterioration in the aging property and a valve opening.According to the present invention, however, the capacitor elementcontains a bonded material formed by bonding a water-soluble metalcomplex to phosphoric acid ions, for which reason this bonded materialcauses phosphoric acid ions discharged into the electrolytic solution,so that a proper amount of the phosphoric acid ions is kept in theelectrolytic solution, and the deterioration of the electrode foilthrough hydration is prevented. Accordingly, the aging property is good.

[0011] Furthermore, if the foil thickness of the anode foil of theabove-described electrolytic capacitor is not less than 70 μm, then theanode foil decreases in resistivity to reduce the ESR, while if the foilthickness of the anode foil is not more than 150 μm, downsizing of thecapacitor is allowable. Thus, it is preferable that the foil thicknessis 70˜150 μm.

[0012] Further, in accordance with the electrolytic capacitor of thepresent invention, the capacitor element provided with the cathode foilusing an etched foil of the void factor of not more than 44% contains abonded material formed by bonding a water-soluble metal complex tophosphoric acid ions and a solvent consisting mainly of water. Bysetting the void factor of the etched foil at 44% or less, the cathodefoil increases in conductivity while the anode foil decreases inresistivity, to thereby achieve, with an assist of the electrolyticsolution using the solvent consisting mainly of water, an electrolyticcapacitor having an unprecedentedly low ESR characteristic. If the voidfactor exceeds 44%, then the cathode foil increases in resistivity,whereby any desirable ESR characteristic is not obtainable. Further, ifthe void factor is 10% or more, then a high capacity is obtained, andthus this is preferable.

[0013] Still further, if the foil thickness of the cathode foil of theabove-described solid electrolytic capacitor is in the rage of 50-100μm, the ESR further decreases and the downsizing of the capacitor isallowable. Thus this is preferable.

[0014] Furthermore, in the above-described solid electrolytic capacitorformed by rolling the electrode foils, the electrode foils of thecapacitor are long and the resistivity of the electrode foils themselvesis essentially large, for which reason the ESR-reducing effect of thecapacitor is extremely large.

[0015] If the metal complex is an aluminum complex, it is possible toform a bonded material including a water-soluble metal complex andphosphoric acid ions within the capacitor element of the aluminumelectrolytic capacitor.

[0016] Moreover, if the content of water in the solvent is 35˜100 wt %,the ESR of the electrolytic capacitor decreases, and thus this ispreferable.

THE BEST MODE FOR CARRYING OUT THE INVENTION

[0017] Embodiments of the present invention will be describedspecifically. An aluminum foil is subjected to an AC-etching in anetching solution such as a hydrochloric acid solution for causing asurface thereof to be rough, and the void factor is set at not more than51%. Further, in order to form a dielectric film, the aluminum foil issubjected to a conversion treatment in a conversion solution such as aphosphoric acid solution to form an anode foil. By setting the voidfactor of the etched foil at not more than 51%, preferably not more than43%, and more preferably not more than 37%, then the anode foilincreases in conductivity and decreases in resistivity, resulting is areduction in the ESR of the electrolytic capacitor. As described above,the present invention reduces the void factor for increasing theconductivity of the electrode foils so as to reduce the ESR, and thusthe effect of the present invention will not decrease, independentlyfrom the conditions for the depth and the area ratio of the etchingpits, for example, even under the condition that the etching pits arcdifferent in depth between the both sides of the etched foil.Furthermore, in order to ensure the maximum capacity per a unit volume,it is preferable that the void factor is not less than 20% to increasethe etching area and obtain the high capacity.

[0018] Moreover, if the thickness of the anode foil is not less than 70μm, and more preferably not less than 90 μm, then the anode foildecreases in resistivity to reduce the ESR. If the thickness of theanode foil is not more than 150 μm, then the capacity per a unit volumeof the capacitor is improved, thereby allowing the downsizing of thecapacitor. Therefore, it is preferable that the foil thickness is 70˜150μm, more specifically 90˜150 μm.

[0019] Further, similarly to the anode foil, another aluminum foil issubjected to the AC etching or the chemical etching for causing asurface thereof to be rough, so that the void factor is not more than44%, preferably not more than 35%, and more preferably not more than 26%o, to thereby form a cathode foil. The cathode foil thus decreases inresistivity to reduce the ESR of the electrolytic capacitor. Stillfurther, in order to ensure the maximum capacity per a unit volume, itis preferable that the void factor is not less than 10% to increase theetching area and obtain the high capacity. Moreover, if the thickness ofthe cathode foil is 50˜100 μm, more specifically 70˜100 μm, then the ESRfurther preferably decreases, thereby allowing the downsizing of thecapacitor.

[0020] In accordance with an electrolytic capacitor using anelectrolytic solution of a low resistivity including a solventconsisting mainly of water for decreasing the void factor and increasingthe conductive part of the electrode foil, optimizations may be made ofa resistance per a unit area of the electrode foil, for example, aresistance defined between opposite sides of the squire-shaped electrodefoil (hereinafter referred to as a resistivity of the foil) as well asof the width and the area of the foil, thereby obtaining a furtherredaction in the ESR of the capacitor.

[0021] The resistivity of the electrode foil used for the presentinvention is 0.4˜0.79 mΩ, more preferably 0.5˜0.61 m Q. This range ofthe electrode foil as used causes the electrode foils to be decreased inresistivity, thereby to reduce the ESR. The lower resistivity than thatrange causes an insufficient effect of reduction to the ESR. The higherresistivity than that range lowers the ESR-redaction effect.

[0022] Further, the foil width is 8˜25 mm, preferably 12˜25 mm. Thenarrower width than this range allows that a reduction of the foilresistivity causes domination of the resistance of the electrolyte,thereby obtaining no reduction in the ESR. The wider width than thisrange decreases the ESR-reduction effect. This is unsuitable for a smallsize product.

[0023] It is necessary that the foil area is not smaller than 1500 mm²,preferably not smaller than 2500 mm², and more preferably not smallerthan 5000 mm². The smaller area of the foil than this range means thatthe contact area between the electrode foils and the electrolyte issmall for making it difficult to reduce the ESR. Further, theelectrolytic capacitor according to the present invention uses thecapacitor element comprises a roll of laminations of the electrode foilssandwiching the separator. This allows the use of the electrode foilshaving a long length for obtaining a sufficient area of the foils anddecreasing the ESR.

[0024] In order that the resistivity of the electrode foil is 0.44˜0.79mΩ, more preferably 0.5˜0.61 mΩ as described above, it is possible touse the electrode foil having a core thickness of 35˜65 μm, morespecifically 45˜55 μm.

[0025] As described above, the cathode foil comprises the etched foil,and the anode foil comprises the etched foil coated with an oxidecoating film which was formed by applying a current to this etched foilin a conversion solution comprising a phosphoric acid solution.Therefore, the cathode comprises an non-etched aluminum part(hereinafter referred to as a core), and the etched part. The anode foilcomprises the non-etched part and the etched part as well as the oxidecoating film. The thickness of the core may be 35˜65 μm, more preferably45˜55 μm, so that the foil resistivity may be 0.4˜0.79 mΩ, morepreferably 0.5˜0.61 mΩ.

[0026] It is also preferable that a conversion film is formed at0.1V˜10V, preferably 0.3˜5V on the cathode foil for reducing the ESR andimproving a life-time at high temperature.

[0027] Also, it is preferable that a layer of anti-oxidizable metalcompound or metal such as titanium nitride or titanium is formed on thesurface of the cathode electrode for increasing an electrostaticcapacity. It is further preferable that the conversion film is formed onthe cathode electrode and the above-described layer of theanti-oxidizable metal or metal compound is further formed on thisconversion film.

[0028] The above-described anode foil and cathode foil are connectedrespectively with electrode plugs and the electrode foils laminatedthrough a separator are rolled. It is possible to obtain the effect ofthe present invention by using the above-described anode foil of thepresent invention and a conventional cathode foil or a conventionalanode foil and the above-described cathode foil of the presentinvention. The maximum effect can be obtained by using both of the anodefoil and the cathode foil of the present invention.

[0029] Usable as a separator in this case are Manila paper, kraft paper,glass separator, nonwoven fabric made of synthetic fiber such as vinylonand polyester, and porous separator.

[0030] When an electrode plug is connected with an electrode foil, ifused is the electrode foil of the present invention with an increasedconductivity, the contact resistance of the jointing of the electrodefoil and the electrode plug decreases to further reduce the ESR of theelectrolytic capacitor.

[0031] Then, the capacitor element thus formed is impregnated with theelectrolytic solution of the present invention using a solventconsisting mainly of water and contained in a cylinder-shapedbottom-closed metal case, so that the opening edge is covered with asealing rubber for sealing by a caulking process.

[0032] The water content of the solvent is 35˜100 wt %. If it is notmore than 75 wt %, the low-temperature characteristic is good, and thusmore preferable is 35˜75 wt %.

[0033] Hereupon, a bonded material formed by bonding a water-solublemetal complex to phosphoric acid ions is included in the capacitorelement. This water-soluble bonded material can be obtained bydissolving in a solvent a compound producing metal ions in a solutioncoupled with a chelating agent (hereinafter, a metal-producing compound)and a compound producing phosphoric acid ions (hereinafter, a phosphoricacid-producing compound). Namely, in this solution, the metal formed ina solution by the chelating agent and the metal-producing compound ischelated to form a water-soluble metal complex. Then, this water-solublemetal complex reacts with the phosphoric acid ion produced in a solutionby the phosphoric acid-producing compound, to thereby form a bondedmaterial formed by bonding a water-soluble metal complex to phosphoricacid ions (hereinafter, a water-soluble bonded material). The solventused here only needs to dissolve the chelating agent, themetal-producing compound, and the phosphoric acid-producing compound.Above all, such materials as water, ethylene glycol and γ-butyrolactoneare preferable. The water-soluble bonded material thus formed is thenadded to the electrolytic solution to be included into the capacitorelement. Also, this water-soluble bonded material may be adhered to theelectrode foils or the separator by an application thereof, so that thewater-soluble bonded material is included in the capacitor element.

[0034] Further, by impregnating the capacitor element with anelectrolytic solution to which the chelating agent, the metal-producingcompound and the phosphoric acid-producing compound are added, thewater-soluble bonded material may be included in the capacitor element.In this electrolytic solution, a water-soluble bonded material is formedby the reaction similar to the above-described reaction in the solvent,and by impregnating the capacitor element with this electrolyticsolution, the water-soluble bonded material may be included in thecapacitor element. Furthermore, in case of an electrode foil comprisingaluminum, aluminum ions are dissolved from the electrode foil, for whichreason it is possible to form a bonded material formed by bonding awater-soluble aluminum complex to phosphoric acid ions without the needof adding a metal-producing compound, and thus it is preferable.

[0035] The water-soluble bonded material thus included graduallydischarge phosphoric acid ions into the electrolytic solution, so thatthe phosphoric acid ions in the electrolytic solution are kept at aproper amount for a long time. Then, by the phosphoric acid ions kept atthe proper amount, the aging property is kept favorably.

[0036] As chelating agents, there are available: α-hydroxycarboxylicacids such as a citric acid, a tartaric acid, a gluconic acid, a malicacid, a lactic acid, a glycolic acid, an α-hydroxybutyric acid, ahydroxymalonic acid, an α-methylmalic acid and a dihydroxytartaric acid;aromatic hydroxycarboxylic acids such as a γ-resorcylic acid,β-resorcylic acid, a trihydroxybenzoic acid, a hydroxyphthalic acid, adihydroxyphthalic acid, a phenol tricarboxylic acid, anaurintricarboxylic acid and an erio chrome cyanine R; a sulfocarboxylicacids such as a sulfosalicylic acid guanidines such as dicyandiamide;sugars such as galactose and glucose lignins such as lignosulfonate, andamino-polycarboxylic acids such as an ethylene diamine tetraacetic acid(EDTA), a nitrilotriacetic acid (NTA), a glycol ether diaminetetraacetic acid (GEDTA), a diethylenetriamine pentaacetic acid (DTPA),a hydroxyethyl ethylene diamine triacetic acid (HEDTA) and a triethylenetetramine hexaacetic acid (TTHA) or these salts. As such salts, anammonium salt, an aluminum salt, a sodium salt and a potassium salt canbe used.

[0037] As metal-producing compounds, there are metals and metalcompounds. As metals, those metals which form a complex coupled with achelating agent such as aluminum, iron, copper, nickel, manganese, zinc,calcium, magnesium, barium, lead, titanium, niobium and tantalum can beused. Further, as metal-producing compounds, those compounds whichproduce metal ions in a solution such as oxide, hydroxide, chloride andmetal salts including sulfate and carbonate can be used. Above all,aluminum is preferable.

[0038] As phosphoric acid-producing compounds, there are the phosphoruscompounds represented by a general formula (Chem. 1) or the saltsthereof, the condensate thereof, the salts of these condensate.

[0039] (In the formula, R₁ and R₂ are —H, —OH, —R₃ and —OR₃: R₃ and R₄are alkyl group, allyl group, phenyl group and ether group.)

[0040] As these phosphoric acid-producing compounds, there are thefollowings: orthophosphoric acid, phosphorous acid and hypophosphorousacid, and the salts thereof. The salts thereof are ammonium salt,aluminum salt, sodium salt, calcium salt and potassium salt.Orthophosphoric acid and the salt thereof dissolve in a solution togenerate phosphoric acid ions. Further, phosphorous acid,hypophosphorous acid and the salts thereof are decomposed in a solutionto generate phosphorous acid ions and hypophosphorous acid ions and thenoxidized to become phosphoric acid ions.

[0041] There are further phosphoric acid compounds such as ethylphosphate, diethyl phosphate, butyl phosphate and dibutyl phosphate, andphosphonic acid compounds such as 1-hydroxy ethylidene-1,1-diphosphonicacid, aminotrimethylene phosphonic acid and phenyl phosphonic acid. Toadd more, there are phosphinic acid compounds such as methyl phosphinicacid and phosphinic acid butyl.

[0042] Further, there are condensed phosphoric acids and the saltsthereof, as follows: linear condensed phosphoric acids such aspyrophoric acid, tripolyphosphoric acid and tetrapolyphosphoric acid,cyclic condensed phosphoric acids such as metaphosphoric acid andhexametaphosphoric acid, and bonded materials of these linear and cycliccondensed phosphoric acids. As salts of these condensed phosphoricacids, ammonium salt, aluminum salt, sodium salt, calcium salt andpotassium salt can be used.

[0043] They are also phosphoric acid-producing compounds which generatephosphoric acid ions in a solution or which generate phosphorous acidions and hypophosphorous acid ions and then are oxidized to becomephosphoric acid ions.

[0044] Preferable among these are orthophosphoric acid which easilygenerates phosphoric acid ions, the salt thereof, condensed phosphoricacid and phosphoric acid compound. Also preferable are orthophosphoricacids which exhibit relatively quick generations of a large number ofphosphoric acid ions for the amount of addition, for example, linearcondensed phosphoric acids such as pyrophoric acid and tripolyphosphoricacid, and the salts thereof. Even with materials other than thesecompounds, the effect of the present invention can be obtained, if thematerials generate phosphoric acid ions in a solution.

[0045] Furthermore, as solutes included in the electrolytic solution,there are ammonium salt, amine salt, quaternary ammonium salt andquaternary salt of a cyclic amidine compound, which are usually used forthe electrolytic solution for electrolytic capacitor, wherein theacid-conjugate base of theses is of anion component. As aminescomprising the amine salt, there are primary amines (such asmethylamine, ethylamine, propylamine, butylamine and ethylene diamine),secondary amines (such as dimethylamine, diethylamine, dipropylamine,methyl ethylamine and diphenylamine) and tertiary amine (such astrimethylamine, triethylamine, tripropylamine, triphenylamine and1,8-diazabicyclo(5,4,0)-undecene-7). As quaternary ammoniums comprisingquaternary ammonium salt, there are tetra-alkyl ammoniums (such as tetramethyl ammonium, tetra ethyl ammonium, tetra propyl ammonium, tetrabutyl ammonium, methyl triethyl ammonium and dimethylethyl ammonium) andpyridiums (such as 1-methyl pyridium, 1-ethyl pyridium and 1,3-diethylpyridium). Further, as cations comprising quaternary salt of a cyclicamidine compound, there are cations formed by quaternarizing thefollowing compounds: namely, imidazole monocyclic compounds (imidazolecongeners such as 1-methylimidazole, 1,2-dimethylimidazole,1,4-dimethyl-2-ethylimidazole and 1-phenylimidazole, oxyalkylderivatives such as 1-methyl-2-oxymethyl imidazole and1-methyl-2-oxyethyl imidazole, nitro derivatives and amino derivativessuch as 1-methyl-4(5)-nitroimidazole and1,2-dimethyl-5(4)-aminoimidazole), benzoimidazoles (such as 1-methylbenzoimidazole and 1-methyl-2-benzyl benzoimidazole), compounds having a2-imidazolin ring (such as 1-methylimidazolin, 1,2-dimethylimidazolin,1,2,4-trimethylimidazolin, 1,4-dimethyl-2-ethylimidazolin and1-methyl-2-phenylimidazolin), compounds having a tetrahydropyrimidinering (such as 1-methyl-1,4,5,6-tetrahydropyrimidine,1,2-dimethyl-1,4,5,6-tetrahydropyrimidine,1,8-diazabicyclo[5,4,0]undecene-7 and 1,5-diazabicyclo[4,3,0]nonen-5).

[0046] As anion components, there are such organic acids asdecandicarboxylic acids including adipic acid, glutaric acid, succinicacid, benzoic acid, isophthalic acid, phthalic acid, terephthalic acid,maleic acid, toluic acid, enanthic acid, malonic acid, formic acid,1,6-decandicarboxylic acid and 5,6-decandicarboxylic acid,octane-dicarboxylic acids including 1,7-octane-dicarboxylic acid,azelaic acids and sebacic acids, or conjugate bases of inorganic acidssuch as boric acid, multiple alcohol complex compound obtained fromboric acid and multiple alcohol, phosphoric acid, carbonic acid andsilic acid. Preferable among these are organic carboxylic acids such asdecandicarboxylic acid, octane-dicarboxylic acid, azelaic acid, sebacicacid, adipic acid, glutaric acid, succinic acid, benzoic acid,isophthalic acid and formic acid, or boric acid and multiple alcoholcomplex compound of boric acid.

[0047] If adipic acid or at least a kind of the salt thereof is used asa solute, the ESR further decreases. The content of the adipic acid orat least a kind of the salt thereof in the electrolytic solution is 5˜23wt %, preferably 8˜18 wt %. With the content above this range, theresistivity decreases, while with the content below this range, thelow-temperature characteristic is good. The content of the other solutesdescribed above in the total electrolytic solution is also approximately5˜23 wt %, preferably 8˜18 wt %.

[0048] In order to further reduce the ESR, it is preferable to useformic acid or the salt thereof as a solute. The content of these in theelectrolytic solution is 3˜15 wt %, preferably 6˜12 wt %. With thecontent below this range, the ESR-reducing effect is small, while withthe content above this range, expansion and opening of a valve arecaused by generation of gas. Further, if an organic acid of 3˜15 wt % orthe salt thereof is added, the ESR decreases. As this organic acid,there are such organic acids as the above-described adipic acid andglutaric acid. Preferable among these is adipic acid.

[0049] As the electrolytic solution of the present invention, a solventconsisting mainly of water is used, and as a secondary solvent, proticpolar solvent, aprotic polar solvent, water and mixture of these may beused. As protic polar solvents, there are monohydric alcohols (such asmethanol, ethanol, propanol, butanol, hexanol, cyclohexanol,cyclopentanol and benzyl alcohol) and multiple alcohols and oxyalcoholcompounds (multiple alcohol compounds and oxyalcohol compounds) (such asethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethylcellosolve, 1,3-butanediol and methoxypropylene glycol). As aproticpolar solvents, there are, as typical examples, amides (such asN-methylformamide, N,N-dimethylformamide, N-ethylformamide,N,N-dimethylformamide, N-methylacetamide and hexamethylphosphoricamide), lactones (such as γ-butyrolactone and δ-valerolactone), cyclicamides (such as N-methyl-2-pyrrolidone), carbonates (such as ethylenecarbonate and propylene carbonate), nitriles (such as acetonitrile),oxides (such as dimethyl sulfoxide), 2-imidazolidinone systems[1,3-dialkyl-2-imidazolidinone (such as 1,3-dimethyl-2-imidazolidinone,1,3-diethyl-2-imidazolidinone and 1,3-di(n-propyl)-2-imidazolidinone)and 1,3,4-trialkyl-2-imidazolidinone (such as1,3,4-trimethyl-2-imidazolidinone)].

[0050] Further, for the purpose of stabilizing the operating-lifecharacteristic of the electrolytic capacitor, it is possible to addaromatic nitro compounds such as nitrophenol, nitro benzoic acid, nitroacetophenone, nitro benzyl alcohol, 2-(nitrophenoxy) ethanol,nitroanisole, nitrophenetol, nitro toluene and dinitrobenzene.

[0051] Furthermore, for the purpose of improving the safety of theelectrolytic capacitor, it is also possible to add a nonionicsurface-active agent cable of improving the withstand voltage of theelectrolytic solution, polyoxyalkylene multiple alcohol ethers obtainedfrom the addition condensation of multiple alcohol to ethylene oxideand/or propylene oxide, and polyvinyl alcohol.

[0052] Moreover, it is possible to further improve the withstand voltageby adding such materials as boric acid, complex carbohydrate (such asmannit, sorbit and pentaerythritol), a complex compound of boric acidwith complex carbohydrate, and colloidal silica, to the electrolyticsolution to be used for the electrolytic capacitor of the presentinvention.

[0053] For the purpose of reducing leakage current, it is furtherpossible to add such materials as oxycarboxylic acid compound.

[0054] The electrolytic capacitor thus formed in accordance with thepresent invention has a low ESR characteristic and a good agingproperty.

[0055] Specific examples of the electrolytic capacitor of the presentinvention will be then described below.

EXAMPLE 1

[0056] An aluminum foil is subjected to an alternate etching for surfaceroughening and then to a conversion for forming a dielectric oxide film,to thereby form the anode foil of the present invention. Further,another aluminum foil is similarly subjected to an alternate etching forsurface roughening, so that a conversion film is formed on the surfacethereof to form a cathode foil. The anode foil and the cathode foil arethen rolled, with the foils laminated through a separator, to therebyform a capacitor element.

[0057] The capacitor element formed as described above is impregnatedwith the electrolytic solution for driving of an aluminum electrolyticcapacitor electrolytic solution. The capacitor element impregnated withthis electrolytic solution is then contained in a cylinder-shapedbottom-closed aluminum outer case, and a sealing member made of butylrubber is inserted in the opening edge of the outer case. Then, theopening edge of the outer case is subjected to a closing process forsealing the aluminum electrolytic capacitor. By so doing, anelectrolytic capacitor of 10Φ×20 L is formed.

[0058] The electrolytic solution used here was produced as follows.First, 10 parts of water is added with 1% of diethylenetriaminepentaacetic acid, 0.2% of aluminum hydroxide and 1.5% of ammoniumdihydrogen phosphate for completing a chelating reaction and aphosphoric acid ion bonding reaction, so as to form a water-solublebonded material. Subsequently, a solution of this water-soluble bondedmaterial is added to an electrolytic solution comprising 50% of water,18% of ethylene glycol, 10% of ammonium adipate and 8% of ammoniumformate, so as to form the electrolytic solution of the presentinvention.

EXAMPLE 1-1

[0059] For an electrolytic capacitor formed as described above, a foilhaving a thickness of 100 μm and a post-etching void factor of 26% wasused as an anode foil and a foil having a thickness of 85 μm and apost-etching void factor of 19% was used as a cathode foil, so as to usethe electrolytic capacitor as the example 1-1.

EXAMPLE 1-2

[0060] For another electrolytic capacitor formed as described above, afoil having a thickness of 100 μm and a post-etching void factor of 46%was used as an anode foil and a foil having a thickness of 50 μm and apost-etching void factor of 39% was used as a cathode foil, so as to usethe electrolytic capacitor as the example 1-2.

EXAMPLE 1-3

[0061] Still another electrolytic capacitor was formed by the samemeasure used for the example 1-2, except that a foil having a thicknessof 100 μm and a post-etching void factor of 33% was used as an anodefoil, so as to use the electrolytic capacitor as the example 1-3.

EXAMPLE 1-4

[0062] For yet another electrolytic capacitor formed as described above,a foil having a thickness of 100 μm and a post-etching void factor of52% was used as an anode foil and a foil having a thickness of 75 μm anda post-etching void factor of 22% was used as a cathode foil, so as touse the electrolytic capacitor as the example 1-4.

COMPARATIVE EXAMPLE 1

[0063] Further, for another electrolytic capacitor formed as describedabove, a foil having a thickness of 100 μm and a post-etching voidfactor of 52% was used as an anode foil and a foil having a thickness of50 μm and a post-etching void factor of 39% was used as a cathode foil,so as to use the electrolytic capacitor as the comparative example 1.

EXAMPLE 2

[0064] Similarly to the example 1, an electrolytic capacitor of 10Φ×12.5L was formed. Then, the electrolytic capacitors having the sameelectrode foils as those of the examples 1-1 and 1-3 were used for theexamples 2-1 and 2-2 respectively, and the electrolytic capacitor havingthe same electrode foils as those of the comparative example 1 was usedfor the comparative example 2.

[0065] (Table 1) represents the comparisons between the foil resistancesof the anode foils and the ESRs of the electrolytic capacitors of theexamples 1-1˜1-4, 2-1 and 2-2, and the comparative examples 1 and 2.TABLE 1 Anode foil Cathode foil Void Foil Foil Void Foil factorthickness resistance factor thickness ESR (%) (μm) (mΩ/m) (%) (μm) (mΩ)Example 1-1 26 100 34 19 85 8.7 Example 1-2 46 100 68 39 50 10.4 Example1-3 33 100 40 39 50 9.9 Example 1-4 52 100 103 22 75 9.9 Example 2-1 26100 70 19 85 16.2 Example 2-2 33 100 84 39 50 19.5 Comparative 52 100103 39 50 11.3 example 1 Comparative 52 100 213 39 50 20.8 example 2

[0066] As can be seen from (Table 1), according to the electrolyticcapacitor of 10Φ×12.5 L, the ESR is reduced in the examples 1-2 and 1-3using the anode foil of the present invention and in the example 1-4using the cathode foil of the present invention, compared with thecomparative example 1. Further, in the example 1-1 wherein an anode foilhaving a void factor of 26% and a cathode foil having a void factor of19% are used, the ESR is reduced to 8.7 m Ω, achieving anunprecedentedly low ESR. Still further, the ESR decreases more in theexample 1-3 wherein an anode foil having a void factor of 33% is usedthan in the example 1-2 wherein an anode foil having a void factor of46% is used.

[0067] Furthermore, as the void factor is reduced from 52% of thecomparative examples to 26, 33 and 46% of the examples, the foilresistance is reduced from 103 mΩ/m to 34˜68 mΩ/m in the example 1, andfrom 213 mΩ/m to 70˜84 mΩ/m in the example 2. This reveals that anelectrode foil increases in conductivity and decreases in resistivity.

[0068] As a comparative example, an electrolytic element was formed in asimilar way to the example 1-1 and the electrolytic element wasimpregnated with a conventional electrolytic solution comprising asolute of quaternary amidinium salt, to thereby form an electrolyticcapacitor. The used electrolytic solution comprises 75 parts ofγ-butyrolactone and 25 parts of ethyl phthalate-dimethyl-imidazolium.The obtained ESR is a high value of 29 m Q, which reveals that theeffect of the present invention cannot be obtained even with the use ofthe electrode foils of the present invention, if an example consistingmainly of water is not used.

[0069] The same result was obtained also from the electrolytic capacitorof 10Φ×12.5 L, which clearly shows the effect of the present invention.

[0070] Subsequently, a no-load test and a high-temperature-load test atthe temperature of 105° C. for 1000 hours were carried out for each ofthe examples 1-1˜1-4, 2-1, 2-2 and a conventional example using theelectrode foils of the comparative example 1 and a conventionalelectrolytic solution to which the water-soluble bonded material of thepresent invention was not added. While good values were obtained fromthe examples, the valves were all opened in a few hours after the startof a test in the conventional example. It was thus found that, inaccordance with the electrolytic capacitor of the present invention, theaging property is good even though the electrolytic capacitor contains asolvent consisting mainly of water.

[0071] The examples specifying the foil resistivity, the foil width andthe foil area will be then described.

[0072] An aluminum foil is subjected to an alternate etching for surfaceroughening and then to conversion for forming a derivative oxide film,to thereby form an anode foil of the present invention. Further, anotheraluminum foil is similarly subjected to an alternate etching for surfaceroughening, so that a conversion film is formed on the surface thereofto thereby form a cathode foil. The anode foil and the cathode foil arerolled, with the foils laminated through a separator, to thereby form acapacitor element,

[0073] The capacitor element formed as described above is impregnatedwith the electrolytic solution for driving of an aluminum electrolyticcapacitor. The capacitor element impregnated with this electrolyticsolution is then contained in a cylinder-shaped bottom-closed aluminumouter case, and a sealing member made of butyl rubber is inserted in theopening edge of the outer case. Then, the opening edge of the outer caseis subjected to a closing process for sealing the aluminum electrolyticcapacitor.

[0074] The electrolytic solution sued here was produced as follows.First, 10% of water is added with 1% of diethylenetriamine pentaaceticacid, 0.2% of aluminum hydroxide and 1.5% of ammonium dihydrogenphosphate for completing a chelating reaction and a phosphoric acid ionbonding reaction, so that a water-soluble bonded material is formed.Subsequently, a solution of this water-soluble bonded material is addedto an electrolytic solution comprising 50% of water, 18% of ethyleneglycol, 10% of ammonium adipate and 8% of ammonium formate, so as toform an electrolytic solution of the present invention.

[0075] (Table 2) represents the foil resistivity, the foil width, thefoil area and the left-core thickness of each of the electrode foilsused here as well as the ESR of the respective examples. TABLE 2 FoilFoil Foil Left-core resistivity width area thickness ESR (mΩ) (mm) (mm²)(μm) (mΩ) Example 1 0.69 9.5 2400 40 12.4 Example 2 0.68 13 1950 41 12.8Example 3 0.55 14 3200 52 8.9 Example 4 0.34 17 2800 51 8.4 Example 50.56 25 5700 50 5.7 Comparative example 1 0.93 10 2700 31 13.9Comparative example 2 0.42 6 3900 65 18.8 Comparative example 3 0.43 141200 65 17.5

[0076] As can be seen from (Table 2), the electrolytic capacitors of theexamples 1-5 of the present invention have a low ESR value of 5.7˜12.8mΩ. Particularly, in the examples 3˜4 with a foil area of not smallerthan 2500 mm², the ESR value is 8.9˜8.4 mΩ or not more than 10 mΩ, andthe ESR value becomes further smaller in the example 5 wherein the foilarea is not smaller than 5000 mm².

[0077] By contrast, in the comparative example 1 wherein the foilresistivity is 0.93 mΩ, the ESR becomes a large value of 13.9 mΩ, eventhough the example has the foil width of 10 mm and the foil area of 2700mm², which are larger than the foil width and the foil area of theexample 1. Further, in the comparative example 2 with the foil width of6 mm, the ESR is as large as 18.8 mΩ, even though the foil resistivityof the example is 0.42 mΩ, which is smaller than the foil resistivitysof the examples 1˜3, and also the foil area of the example is 3900 mm²,which is larger than the foil areas of the examples 1˜3. Still further,in the comparative example 3 with the foil area of 1200 mm², the ESRbecomes a large value of 17.5 mΩ, despite a low foil resistivity of 0.43mΩ.

[0078] Furthermore, as a comparative example 4, an capacitor element wasformed in a similar way to the example 1 and then impregnated with aconventional electrolytic solution comprising a solute of quaternaryamidinium salt, to thereby form an electrolytic capacitor. The usedelectrolytic solution comprises 75% of γ-butyrolactone and 25% of ethylphthalate-dimethyl-imidazolium. The obtained ESR is a high value of 29.1mΩ, which reveals that the effect of the present invention cannot beobtained even with the use of the electrode foils of the presentinvention, if an example consisting mainly of water is not used.

[0079] Subsequently, a no-load test and a high-temperature-load test atthe temperature of 105° C. for 1000 hours were carried out for each ofthe electrolytic capacitors of the examples 1˜3 and the electrolyticcapacitors using the electrode foils of the examples 1-3 and aconventional electrolytic solution to which the water-soluble bondedmaterial of the present invention is not added. While good values wereobtained from the examples, the valves were all opened in a few hoursafter the start of the test in the conventional example. It was thusfound that, in accordance with the electrolytic capacitor of the presentinvention, the aging property is good even though the electrolyticcapacitor contains a solvent consisting mainly of water.

[0080] As described above, in accordance with the present invention, itis possible to provide an electrolytic capacitor having anunprecedentedly low ESR characteristic and a good aging property and anelectrode foil for electrolytic capacitor to be used therefor.

1. An electrolytic capacitor containing a solvent within a capacitorelement provided with an anode foil using an etched foil of a voidfactor of not more than 51%, wherein said solvent consists essentiallyof a bonded material comprising a water-soluble metal complex bondedwith phosphoric acid ions and water.
 2. An electrolytic capacitorcontaining a solvent within a capacitor element provided with a cathodefoil using an etched foil of a void factor of not more than 44%, whereinsaid solvent consists essentially of a bonded material comprising awater-soluble metal complex bonded with phosphoric acid ions and water.3. The electrolytic capacitor as claimed in claim 1, wherein said voidfactor is not less than 20%.
 4. The electrolytic capacitor as claimed inclaim 2, wherein said void factor is not less than 10%.
 5. Theelectrolytic capacitor as claimed in claim 1, wherein a thickness of theanode foil is 70˜150 μm.
 6. The electrolytic capacitor as claimed inclaim 2, wherein a thickness of the cathode foil is 50-100 μm.
 7. Theelectrolytic capacitor as claimed in any one of claims 1-6, wherein saidcapacitor element is formed by rolling a lamination structure of aseparator sandwiched between said anode foil with an anode plug and saidcathode foil with a cathode plug.
 8. The electrolytic capacitor asclaimed in any one of claims 1-7, wherein said metal complex is analuminum complex.
 9. The aluminum electrolytic capacitor as claimed inclaims 1-8, wherein a water content of said solvent is 35-100 wt %. 10.An anode foil for electrolytic capacitor, said anode foil comprising anetched foil of a void factor of not more than 51%.
 11. A cathode foilfor electrolytic capacitor, said cathode foil comprising an etched foilof a void factor of not more than 44%.